This invention relates to a process for separating a 3,5-dialkylphenol wherein the alkyl groups are primary or secondary and have a sum total of carbon atoms greater than two from a mixture containing mono-alkylphenols and dialkylphenols other than 3,5-substituted. More particularly, this invention relates to a process for separating 5-isopropyl-m-cresol (m-thymol) from a mixture containing other isopropyl-cresols and possibly some di-isopropylphenols other than 3,5-dialkylphenols.
Some methods of producing a desired compound also involves the production of by-products or isomers. This is especially true in the production of 3,5-dialkylphenols where the alkyl groups are primary or secondary and have a sum total number of carbon atoms greater than two. Most methods for the production of these 3,5-dialkylphenols produce undesired isomers where the alkyl group is attached to the aromatic ring at a position other than the third and fifth positions. One example of such a 3,5-dialkylphenol is 3-isopropyl-5-methylphenol. Most of the methods available for producing this compound also produce undesirable isomers which include: 6-isopropyl-m-cresol; 2-isopropyl-m-cresol; 4-isopropyl-m-cresol; 3-isopropyl-p-cresol; 2,4-di-isopropylphenol; 2,5di-isopropylphenol, and possibly other di-isopropylphenols.
The compound 3-isopropyl-5-methylphenol is useful as an intermediate for preparing insecticides, e.g., its N-methyl carbamate derivative. In addition, the 3-isopropyl-5-methylphenol and other 3,5-dialkyphenols are valuable intermediates for preparing other compounds. It should be noted that 3-isopropyl-5-methylphenol is commonly referred to in the literature by several other names. In addition to 3-isopropyl-5-methylphenol, the compound has been referred to as 5-isopropyl-m-cresol, symmetrical thymol (sym-thymol), 5-thymol, or m-thymol. It was postulated at a symposium on "Carbamate Insecticides" at the 148th ACS meeting in the fall of 1964 that the order of insecticidial activity of alkylphenol-N-methyl carbamate was m-alkyl&gt;o-alkyl or para-alkyl, sec-alkyl&gt;tert-alkyl&gt;n-alkyl and 4-carbon side chain&gt;3 or 5carbon&gt;fewer or more carbons. Therefore, the N-methyl carbamate of 5-isopropyl-m-cresol would be a very effective insecticide. In fact, the N-methyl carbamate of this compound has been patented for this use. See Jaeger and Teissker, German Pat. No. 1,147,438.
Alkylation of m-cresol with propylene or isomerization or transalkylation of other isopropyl-m-cresols produce a mixture of the isomers of 5-isopropyl-m-cresol and similar compounds. These compounds include mono- or poly-isopropyl-m-cresols as well as di-isopropylphenols and unalkylated cresols. When m-cresol is alkylated with propylene a mixture is obtained that contains 5-isopropyl-m-cresol and the following compounds: m-cresol, 6-isopropyl-m-cresol (thymol); 4-isopropyl-m-cresol (p-thymol); 2-isopropyl-p-cresol; 3-isopropyl-p-cresol and possibly 2,4-di-isopropylphenol and 2,5-di-isopropylphenol. Some of the compounds in this mixture have boiling points that are considerably different from the boiling point of 5-isopropyl-m-cresol. Therefore, these compounds can be easily removed from the mixture by fractional distillation. These compounds are 2-isopropyl-p-cresol, and 6-isopropyl-m-cresol. As can be seen from the boiling points listed in Table I below, complete separation of 5-isopropyl-m-cresol from the other compounds in the mixture having similar boiling points is extremely difficult.
Table I ______________________________________ Boiling Points .degree. C at 760 mm Hg ______________________________________ Thymol (6-isopropyl-m-cresol) 233 2-isopropyl-p-cresol 228-233 m-thymol (5-isopropyl-m-cresol) 241 p-thymol (4-isopropyl-m-cresol) 245-246 3-isopropyl-p-cresol 245 2,4-di-isopropylphenol 248 2,5-di-isopropylphenol 248.sup.(1) ______________________________________ .sup.(1) 2,5-di-isopropylphenol has no normal b.p. given in the literatur but it should boil around the same temperature as its 2,4 isomer, i.e., 248.degree. C.
Compounds which boil at a lower temperature than m-thymol can be separated from the mixture and from m-thymol by fractional distillation. Also, these compounds can be separated readily from m-thymol by the process of this invention. Therefore, these compounds need not be removed from the mixture in order to separate m-thymol from the mixture. Of the compounds in Table I having a higher boiling point than m-thymol, p-thymol and 3-isopropyl-p-cresol have boiling points that are only slightly higher than the boiling point of m-thymol. These small differences are not enough to enable these compounds to be separated by distillation from the m-thymol. Regardless of the method of distillation, these compounds still will be present in any m-thymol boiling fraction of the mixture. By the process of the present invention m-thymol can be separated easily and efficiently from a mixture containing the compunds in Table I.
In the German Offenlegungsschrift No. 2,340,218, there is disclosed a method for recovering m-thymol from a mixture of isopropyl-cresols. This method involves alkylating the mixture of isopropylcresols with an alkylating agent so that the 5-isopropyl-m-cresol is not alkylated. The 5isopropyl-m-cresol is then separated from the mixture of alkylated-isopropyl-cresols by fractional distillation or by extraction when 2,6-di-isopropyl-p-cresol is present in the mixture of isopropyl-cresols. This selective alkylation and separation effectively purifies the 5-isopropyl-m-cresol from the mixture of alkylated-isopropyl-cresols. This method does not mention any possible use for the mixture of alkylated-isopropylcresols after m-thymol is separated from it. This mixture of alkylated-isopropylcresols would be best utilized if it could be isomerized to thymol or m-thymol. Before such an isomerization could take place in the above method, the alkylated-isopropylcresol mixture would have to be dealkylated. Such a dealkylation of this mixture would be difficult to perform because of the presence of the methyl group on the benzene ring. Any dealkylation of the alkylated isopropylcresols would probably remove both alkyl groups, the added alkyl group and the isopropyl group, from the benzene ring. Furthermore, isomerization of the methyl groups and transalkylation from one phenol nucleus to another would occur. Therefore, the production of any thymol or m-thymol by the isomerization of a mixture of alkylated-isopropylcresols would be troublesome.
Another method used in the art to separate compounds having similar boiling points is sulfonation. The art gives conflicting reports on whether or not compounds like m-thymol can be sulfonated. German Pat. No. 283,306 by F. Raschig in 1914 showed that 3,5-xylenol, a compound similar to m-thymol, sulfonated by treating it with sulfuric acid. This sulfonation yields 1,3-dimethyl-5-hydroxy-4-sulfonic acid. In 1928 Horst Bruckner in Z. ANAL. CHEM. 75:289-92, while trying to separate meta-, and para-cresols and other phenols by sulfonation, determined that only one phenol, 3,5-xylenol would not sulfonate with concentrated sulfuric acid.
In U.S. Pat. No. 2,840,616, entitled "Production of Thymol", there is disclosed a method of converting thymol isomers to thymol. This method involves: Sulfonating thymol isomers, isomerizing the resulting sulfonic acid, desulfonating the product of the isomerization to produce a mixture of thymol and thymol isomers and recovering the thymol from the thymol isomers. A thymol isomer is defined as a compound having a general formula of m-cresol with an isopropyl radical located at one of the positions other than that ortho to the hydroxyl radical and para to the methyl radical in the benzene ring. The sulfonation of these thymol isomers is conducted with two or more moles of concentrated sulfuric acid or fuming sulfuric acid at a temperature near 100.degree. C. This disclosure indicates that m-thymol is sulfonated with the other thymol isomers to produce the sulfonic acid of the thymol isomers.